ORDER I PRIONODESMACEA 377 
in front of the anterior cardinal; posterior scar longer, less distinct, situated on an 
elevated or thickened radial ridge. The oldest Devonian species (MZ. cucullatus, Goldt.) 
has amorphous cardinals and a smooth rounded shell (Humegalodon). The Triassic 
species sometimes attain a large size, have a radial posterior ridge, smooth teeth, and 
divided right posterior cardinal teeth (Neomegalodon, Giimb.) They are extraordinarily 
abundant in the Dachstein, Limestone, and Hauptdolomite of the Northern Alps, and 
are also plentiful in the Raibl and Rhaetic beds of the Southern Alps. 
Pachyrisma, Morr. and Lye. (Pachymegalodon, Giimb.) Like Megalodon, but with a 
larger anterior adductor scar, a rounded anterior tooth before the cardinals, and a 
strong posterior lateral. Trias to Upper Jura. 
Durga, Bohm. Like Pachyrisma, but without an elevated area at the posterior 
adductor. Lias. 
Protodiceras, Bohm. Lias. Dicerocardiwm, Stoppani. Rhaetie. 
Family 28. Unionidae. Fleming. 
Shell equivalve, dimyarian, typically Schizodont, with pseudocardinals and laterals 
if dentiferous ; conspicuously nacreous ; beaks usually sculptured ; ligament opisthodetie, 
external ; lobes of the mantle united to form an anal siphon, but the functional branchial 
siphon usually incomplete below ; foot compressed, keeled, large, rarely with a feeble byssus ; 
usually dioecious ; the young having a distinct nepionie stage (glochidium). Fluviatile. 
Trias to Recent. 
Typical Uniones make their appearance in the Trias of Texas, but are not abundant until 
the Cretaceous and Tertiary. The origin of the family has been sought in the 7'rigoniidae,” 
which have a very similar ontogeny as a group ; in 7rigonodus* and related forms ; and by 
Pohlig in the Triassic Uniona. An older view recognises the Carboniferous Anthracosia and 
other Cardiniidae as probable ancestors. The weight of evidence is in favour of the latter, 
though there is much probability that each of these groups bearsa certain amount of relation- 
ship to the present family, which will be better realised when more evidence is obtainable. 
Unio, Retzius (Fig. 650). This, the typical genus, was founded on the pearl 
mussel (Mya margaritifera, Linn.), in which the posterior laminae of the hinge are 

Fic. 650. 
Unio Stachei, Neumayr. Pliocene (Congerian stage); Sibinj, Slavonia. p, Adductor; x, Pedal sear. 
obsolete. The majority of species, however, have amorphous, heavy, radial, pseudo- 
cardinal and lateral teeth on the hinge; the shell is variable in form and ornamenta- 
tion, some species having strongly marked sexual differences in the shell. Most of 
the species* are pearly, with a conspicuous brown or greenish periostracum; the 
anterior adductor scars are high, and the pedal scars conspicuous. 
Tausch, L. von, Ueber Conchodus, ete. (Abhandl. geol. Reichsanstalt, XVII.), 1892. 
Neumayr, M., Ueber die Herkunft der Unioniden (Sitzber. Wien. Akad. XCVIII.), 1889. 
Wohrmann, S. von, Ueber die systematische Stellung der Trigonidem und die Abstammung der 
Nayaden (Jahrb. geol. Reichsanst. Bd. XLIII.), 1893. 
one 
